FIG. 5 shows an example of a vacuum interrupter, wherein 1 is a vacuum interrupter, and 2 is an insulating tube having both ends to which end plats 3, 4 are mounted to form a vacuum container. A stationary lead 5 having a stationary electrode is arranged through the end plate 3, and a movable lead 7 having a movable electrode is movably arranged through the end plate 4 through a bellows 6. 8 is a shield which is mounted to a middle of the insulating tube to prevent metallic vapor produced between the stationary electrode and the movable electrode from adhering on the inner surface of the insulating tube 2.
Typically, the vacuum interrupter has a normal interrupting capability at the pressure with the degree of vacuum being 5×10−4 Torr or less. However, with a long period of use, the degree of vacuum can be degraded due to gas discharged from the inside of the interrupter, slow leakage from a junction by welding, soldering or the like at the time of manufacturing, etc., causing gradual lowering of the interrupting capability.
Since occurrence of poor interruption has a considerably detrimental effect on a power system having the interrupter arranged therein, monitoring of the degree of vacuum when using the vacuum interrupter forms a significant challenge.
FIG. 6 shows the relationship between the degree of vacuum called Paschen curve and the internal discharge of the vacuum interrupter. If occurrence of poor degree of vacuum leads to more than 5×10−4 Torr, the discharge is produced between the electrode and the shield in the closed-circuit state of the interrupter. Monitoring of the degree of vacuum is based on detection of this discharge which forms a detection principle for degradation of the degree of vacuum.
Various apparatus for monitoring the degree of vacuum based on the above monitoring principle have been proposed, any of which is constructed to detect a frequency of about 2-20 KHz, presenting insufficiency in terms of the detection sensibility.
Specifically, in the neighborhood of the monitoring apparatus for detecting discharge due to degradation of the degree of vacuum, various noises always occur mixedly in addition to the vacuum interrupter, such as noise produced by a pantograph of a train during passage thereof, noise due to lightening surge and switching surge produced at switching of the interrupter, noise due to excitation rush current of a substation transformer and corona discharge from an insulator during raining, etc. These noises occurs discontinuously, which cannot be distinguished from noise due to degradation of the degree of vacuum, resulting in insufficient detection sensibility of the degree of vacuum.